Engine combustion control at low loads via fuel reactivity stratification

ABSTRACT

A compression ignition (diesel) engine uses two or more fuel charges during a combustion cycle, with the fuel charges having two or more reactivities (e.g., different cetane numbers), in order to control the timing and duration of combustion. By appropriately choosing the reactivities of the charges, their relative amounts, and their timing, combustion can be tailored to achieve optimal power output (and thus fuel efficiency), at controlled temperatures (and thus controlled NOx), and with controlled equivalence ratios (and thus controlled soot). At low load and no load (idling) conditions, the aforementioned results are attained by restricting airflow to the combustion chamber during the intake stroke (as by throttling the incoming air at or prior to the combustion chamber&#39;s intake port) so that the cylinder air pressure is below ambient pressure at the start of the compression stroke.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-AC04-94AL85000awarded by the U.S. Department of Energy. The government has certainrights in this invention.

FIELD OF THE INVENTION

This document concerns an invention relating generally to combustionoptimization methods for compression ignition (diesel) engines, and morespecifically to combustion optimization methods resembling thosedescribed in prior related U.S. patent application Ser. No. 12/793,808(filed on Feb. 11, 2010), but adapted for diesel engines operating atlow loads.

BACKGROUND OF THE INVENTION

Diesel (compression ignition) engines are among the mostenergy-efficient engines available, with admirably high power output perfuel consumption. Unfortunately, they're also among the “dirtiest”engines available, with common diesel engines (at the time of thisdocument's preparation) being prone to high production of nitrogenoxides (commonly denoted NOx), which result in adverse effects such assmog and acid rain, and particulates (often simply called “soot”),sometimes seen as the black smoke emitted by a diesel vehicle as itaccelerates from a stop. Soot particularly tends to be a problem whendiesel engines are idling or operating at low loads, and for thisreason, many areas of the United States have adopted “anti-idling” lawslimiting the amount of time that a diesel vehicle can be left idling.

Because of the impact of soot and NOx emissions on the environment, theUnited States and many other countries have also imposed stringentemissions regulations on the use of diesel engines in vehicles, andnumerous technologies have been developed which attempt to reduce dieselemissions. As an example, NOx is generally associated withhigh-temperature engine conditions, and may therefore be reduced by useof measures such as exhaust gas recirculation (EGR), wherein the engineintake air is diluted with relatively inert exhaust gas (generally aftercooling the exhaust gas), thereby reducing the oxygen in the combustionchamber and reducing the maximum combustion temperature. As anotherexample, soot is generally associated with incomplete combustion, andcan therefore be reduced by increasing combustion temperatures, or byproviding more oxygen to promote oxidation of the soot particles.Unfortunately, measures which reduce NOx production in an engine tend toincrease soot production, and measures which reduce soot production inan engine tend to increase NOx production, resulting in what is oftentermed the “soot-NOx tradeoff.”

NOx and soot can also be addressed after they leave the engine (e.g., inthe exhaust stream), but such “after-treatment” methods tend to beexpensive to install and maintain. As examples, the exhaust stream maybe treated with catalysts and/or injections of urea or otherreducing/reacting agents to reduce NOx emissions, and/or fuel canperiodically be injected and ignited in the exhaust stream to burn offsoot collected in “particulate traps.” These approaches requireconsiderable expense and complexity, and in the case of particulatetraps, they tend to reduce a vehicle's fuel efficiency.

Other technologies have more fundamentally focused on how to reduce bothNOx and soot generation from the combustion process and thereby obtaincleaner “engine out” emissions (i.e., emissions directly exiting theengine, prior to exhaust after-treatment or similar measures). Theseapproaches include modifying the timing, rate, and/or shape of fuelinjection charges, modifying the combustion chamber shape, and/ormodifying other factors to try to attain complete combustion of all fuel(and thus lower soot) while controlling the combustion temperature (thuscontrolling NOx). Many of these technologies provide emissionsimprovements, but are difficult to implement and control, particularlyover the complete range of speeds and loads over which common dieselvehicle engines must operate. Additionally, many of these technologiesstill require measures such as exhaust after-treatment to attainemissions targets, leading to the aforementioned issues with cost andfuel efficiency.

Because of the difficulties in complying with emissions regulationswhile providing the fuel efficiency, cost, and performance thatconsumers seek, many automotive companies have simply shifted theirfocus away from diesel engines to the use of gasoline engines. Gasolineengines unfortunately have lower energy efficiency, and their emissionsare also of concern. (For the reader having limited familiarity withinternal combustion engines, the primary difference between gasolineengines and diesel engines is the manner in which combustion isinitiated. Gasoline engines—also commonly referred to as spark ignitionor “SI” engines—provide a relatively fuel-rich mixture of air and fuelinto an engine cylinder, with a spark then igniting the mixture to drivethe piston outwardly from the cylinder to generate work. In dieselengines—also known as compression ignition engines—fuel is introducedinto an engine cylinder as the piston compresses the air therein, withthe fuel then igniting under the compressed high pressure/hightemperature conditions to drive the piston outwardly from the cylinderto generate work.)

A prior patent application by the inventors—U.S. patent application Ser.No. 12/793,808, filed Feb. 11, 2010 (and incorporated by referenceherein)—describes diesel combustion methods, now referred to asReactivity-Controlled Compression Ignition (RCCI) methods, wherein thefuel provided to the engine's combustion chamber is adapted to have itsreactivity vary over the course of a combustion cycle. (“Reactivity” isa property corresponding to a fuel's tendency to spontaneously igniteunder diesel operation conditions, i.e., under high pressures andtemperatures. Thus, reactivity generally corresponds to a fuel's cetanenumber, or the converse of the fuel's octane number). In these RCCImethods, the fuel is also provided to the combustion chamber in such amanner that a stratified distribution of fuel reactivity results, thatis, spaced regions of high reactivity and low reactivity are situatedwithin the combustion chamber during the compression stroke of thecombustion cycle. During compression, the higher-reactivity regionsignite first, with combustion then propagating to the lower-reactivityregions. With appropriate tailoring of fuel reactivity, fuel/reactivityamounts and proportions, the timing of fuel introduction into thecombustion chamber, and similar factors, combustion can be tailored toproduce peak work output at the desired time (for optimal power output),with low NOx and soot production. Experimental engines implementing RCCImethods resulted in exceptionally high fuel efficiency while meetingU.S. government emissions standards applicable at that time, without theneed for exhaust gas after-treatment.

To review preferred versions of the RCCI methods in greater detail, aninitial fuel charge having a first reactivity is supplied to thecombustion chamber during the intake and/or compression stroke,preferably sufficiently early that the initial fuel charge is highlypremixed with the air in the combustion chamber during a major portionof the compression stroke. One or more subsequent fuel charges ofdifferent reactivity are thereafter supplied to the combustion chamberin such a manner that a stratified distribution of fuel reactivityresults within the combustion chamber, with distinct regions of higherand lower fuel reactivity. More specifically, the laterdifferent-reactivity charges are timed and otherwise designed todistribute the different-reactivity charges—which will be introducedinto a preferably highly premixed “matrix” of air and first-reactivityfuel—in such a manner that the reactivity gradient within the combustionchamber provides a desired combustion start time and rate (a time/ratethat results in controlled heat release resulting in superior work inputto the piston), while deterring rapid pressure increases and hightemperatures (which promote NOx production and reduce fuel economy), andwhile completely burning all (or nearly all) fuel within the combustionchamber to reduce unburnt hydrocarbons. Combustion tends to begin in oneor more regions of highest reactivity (these regions being generated viathe introduction of the higher-reactivity material), and spreadstherefrom via volumetric energy release and/or flame propagation untilall fuel from all charges is consumed. Thus, tailoring of the reactivitydistribution within the combustion chamber can allow tailoring of thenature of the combustion process. Greater stratification/gradation inreactivity tends to result in a lower combustion rate. Conversely, lowerstratification/gradation in reactivity (greater uniformity in reactivitythroughout the combustion chamber) tends to result in a highercombustion rate, since each location within the chamber has anapproximately equal chance of igniting first, and those that do notignite first will be rapidly ignited by their neighbors.

The different fuel charges, with their differing reactivities, can beconventional fuels supplied to the engine from separate conventionaltanks, e.g., diesel fuel (which has a higher reactivity) from one tank,and gasoline (which has lower reactivity) from another tank.Alternatively or additionally, fuel from a single tank can have itsreactivity modified between higher and lower levels by the addition ofan appropriate reactivity modifier. As an example, an initiallower-reactivity charge could simply contain gasoline or diesel fuel,and a subsequent higher-reactivity fuel charge could contain thegasoline or diesel fuel with a small amount of Di-Tertiary ButylPeroxide (DTBP), 2-ethyl hexyl nitrate, or another cetane improver. Anarrangement of this nature is useful since many reactivity modifiers areonly needed in very dilute amounts, and thus a smaller tank forcontaining a reactivity modifier could be provided in a vehicle alongwith a conventional fuel tank arrangement, and with a meteringarrangement that mixes a desired amount of reactivity modifier into thefuel line (or into a high-reactivity fuel line separate from alow-reactivity fuel line) when appropriate. To illustrate, aconventional diesel vehicle with a supplementary 1-2 quart tankcontaining DTBP would only require refilling every 3000-6000 miles orso, which is roughly the recommended frequency for an oil change, andthus the reactivity modifier tank could be refilled when the vehicle'soil is changed.

To review the reactivity stratification in greater detail, the initialfirst-reactivity fuel charge is supplied into the combustion chambersufficiently prior to Top Dead Center (TDC) that the initial fuel chargeis at least partially premixed (homogeneously dispersed) within thecombustion chamber before the subsequent injection(s) is/are made. Theinitial charge may be introduced into the combustion chamber via(preferably low-pressure) direct injection into the cylinder, and/or byproviding it through the combustion chamber's intake port, as byinjecting or otherwise introducing the charge into the intake manifold,and/or into an intake runner extending therefrom. A first subsequenthigh-reactivity fuel charge is then supplied to the combustion chamberduring approximately the first half of the compression stroke,preferably between the time the intake port is closed and approximately40 degrees before TDC. More particularly, for a typical combustionchamber which is partially bounded by a piston face with a central bowl,as depicted in FIGS. 1A-1D, the first subsequent fuel charge ispreferably introduced at such a time (and with such pressure) that atleast a major portion of the first subsequent fuel charge is directedtoward an outer (squish) region located at or near an outer radius ofthe piston face. More specifically, the first subsequent fuel charge isdirected toward a region located outside of an outer third of the radiusof the piston face. This is exemplified by FIG. 1B, which shows thecombustion chamber at approximately 60 degrees before TDC, and with aninjection being directed by the injector toward the squish region.However, in all instances injection is always preferably provided atpressures which avoid or minimize charge impingement on combustionchamber surfaces, since such impingement tends to enhance sootproduction.

A second subsequent high-reactivity fuel charge is then supplied to thecombustion chamber after the first subsequent fuel charge. FIG. 1Cdepicts such an injection being made at approximately 30 degrees beforeTDC, with at least a major portion of the injection being directedtoward an inner (bowl) region spaced inwardly from the outer radius ofthe piston face. More specifically, at least a major portion of thesecond subsequent fuel charge is preferably injected toward a regionlocated inside an outer fourth of the radius of the piston face (i.e.,it is injected toward a region defined by the inner 75% of the boreradius). In the meantime, the first subsequent fuel charge has begun todiffuse from the squish region, and to mix with the low-reactivity fuelfrom the initial fuel charge to form a region of intermediate reactivityat or near the squish region.

FIG. 1D then illustrates the combustion chamber of FIG. 1B atapproximately 15 degrees before TDC, with the fuel in the chamber havinga reactivity gradient ranging from higher-reactivity regions in the bowlto lower-reactivity regions at the outer diameter of the chamber, and atthe crown of the bowl. Combustion may begin around this time, startingat the higher-reactivity region(s) and then propagating to thelower-reactivity regions over time.

Basically the same combustion mechanism results if the reactivities ofthe charges of FIGS. 1A-1D are reversed, i.e., if one or more initialhigher-reactivity charges are followed by one or more subsequentlower-reactivity charges: ignition begins in the higher-reactivityregions and propagates to the lower-reactivity regions. The start andduration of combustion can be controlled by the timings and amounts ofthe fuel charges, which affect the degree of stratification attained.For optimal work output, it is desirable that the fuel charges aresupplied to the combustion chamber to attain peak cylinder pressure ator after Top Dead Center (TDC), more preferably between TDC and 20degrees ATDC (After TDC), and most preferably between 5 and 15 degreesATDC. In similar respects, CA50 (i.e., 50% of the total fuel massburned) preferably occurs between approximately 0 to 10 degrees ATDC. Itis also useful to supply the fuel charges in such a manner that the rateof pressure rise is no greater than 10 bar per degree of crank anglerotation, since greater pressure rise can generate unwanted noise andmore rapid engine wear, and also promotes higher temperatures (and thusincreased fuel consumption owing to heat transfer losses, as well as NOxproduction).

Use of the foregoing RCCI methodology tends to result in much lower peakcombustion temperatures—as much as 40% lower—than in conventional dieselengines, owing to the increased control over the combustion process.This deters NOx formation, and additionally increases engine efficiencybecause less energy loss occurs from the engine through heat transfer.Further, the reactivities, amounts, and timing of the fuel charges canbe adapted to optimize combustion such that there is less unburned fuelleft at the end of the expansion stroke (and thus lost to the exhaust),thereby also enhancing engine efficiency, and also generating less soot.

Experimental results of the RCCI methods operating with diesel andgasoline fuels yielded a net indicated thermal efficiency of up to 53%,and a gross thermal efficiency of about 56%. (Thermal efficiency is auseful measure of fuel efficiency, as it represents the amount of fuelconverted to output power by the engine, as opposed to being lost viaheat transfer, exhaust, or other variables. Net thermal efficiency takesaccount of work output over the entire engine cycle, whereas grossthermal efficiency only takes account of the expansion and compressionstrokes, with an approximately 3% difference between the two beingcommon.) In contrast, at the time the RCCI methods were first developed,the average conventional diesel engine had a thermal efficiency ofapproximately 42%, and the average gasoline engine had a thermalefficiency of approximately 25-30%. In short, the RCCI methods yieldedexceptionally high fuel efficiency. At the same time, they met U.S.governmental soot emissions limits, NOx emissions limits, and fuelconsumption limits for the year 2010 without the need for exhaust gasafter-treatment. Emissions could be lowered even further with theimplementation of measures such as exhaust after-treatment.

However, further experimentation found that with decreasing engine load,RCCI methods did not function as well. As noted in the priorapplication, in versions of the invention using diesel fuel andgasoline, the invention required greater amounts of (higher-reactivity)diesel fuel and lesser amounts of (lower-reactivity) gasoline as loaddecreased. Below loads of approximately 4 bar IMEP, and particularly atidle (i.e., less than about 1 bar), the engine effectively operated as aconventional diesel engine, with minimal or no use of gasoline and onlyusing diesel fuel. This yielded conventional low-load dieselperformance, i.e., lower thermal efficiency and undesirably highemissions. Since many diesel engine applications—most notably automotiveapplications—require idling and other low-load operation, it isdesirable to develop adaptations to the foregoing RCCI methods thatallow low-load operation while using the fuel reactivity stratificationdescribed above—and yielding its benefits as described above—rather thanoperating as a conventional diesel engine with typical (and undesirable)diesel emissions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D schematically illustrate the cross-sectional area of acombustion chamber of a diesel (compression ignition) engine as itspiston moves from a position at or near Bottom Dead Center (FIG. 1A) toa position at or near Top Dead Center (FIG. 1D), showing an enginecombustion method exemplifying the invention wherein a firstlow-reactivity fuel charge is already at least substantiallyhomogeneously dispersed within the chamber in FIG. 1A, a firstsubsequent high-reactivity fuel charge is injected into the chamber inFIG. 1B, and a second subsequent high-reactivity fuel charge is injectedinto the chamber in FIG. 1C.

FIG. 2 is a simplified depiction of a diesel engine suitable forpracticing the invention, wherein a throttle 222 is used to restrict airintake such that the combustion chamber 204 contains air atsub-atmospheric pressure at the start of the compression stroke.

FIG. 3A is an exemplary plot of load vs. speed for an engine such asthat of FIG. 2, illustrating the conditions at which the inventionoperates (as the cross-hatched region).

FIG. 3B is an exemplary plot of equivalence ratio vs. speed for anengine such as that of FIG. 2, again illustrating the conditions atwhich the invention operates, with sub-atmospheric combustion chambercontents preferably being used when an equivalence ratio of less thanapproximately 0.4 is required.

DETAILED DESCRIPTION OF EXEMPLARY VERSIONS OF THE INVENTION

The invention, which is defined by the claims set forth at the end ofthis document, is directed to diesel engines and diesel combustionmethods which allow use of the aforementioned fuel reactivitystratification at low loads, and which at least partially provide theaforementioned beneficial results of reactivity stratification.Following is a brief summary of preferred versions of the invention,with more details being provided elsewhere in this document.

FIG. 2 schematically depicts an exemplary engine cylinder 200 bearing areciprocating piston 202 (the piston 202 having a domed face shown as aphantom/segmented line), with a combustion chamber 204 being situatedbetween the piston 202 and the cylinder head 206. An intake manifold 208opens onto the combustion chamber 204 at an intake port 210 bearing anintake valve 212. Similarly, an exhaust valve 214 is openable andclosable within an exhaust port 216 opening onto the combustion chamber204, with the exhaust port 216 leading to an exhaust manifold 218. Tanks220 and 222—which, like the other elements shown in FIG. 2, are onlyillustrated in conceptual form rather than in their true shapes,proportions, and locations—contain materials (fuels and/or fueladditives) having different reactivities, e.g., gasoline in one tank anddiesel fuel in the other, gasoline in one tank and a cetane improver inthe other, or other arrangements. These materials are supplied to thecombustion chamber 204 (possibly after premixing) as fuel charges via afuel injector 224 is situated in the cylinder head 206, and/or via afuel injector 226 upstream from the intake port 210. The materials fromthe tanks 220 and 222 can be metered to one or both of the fuelinjectors 224 and 226 in any appropriate fashion, e.g., the fuelinjector can first inject a gasoline charge followed by a diesel charge,or can first inject a gasoline charge followed by a gasoline chargecontaining cetane improver, etc.

As noted previously in this document (and in the prior patentapplication), charges from the tanks 220 and 222 can be injected intothe combustion chamber 204 with timings and fuel amounts that result ina stratified reactivity distribution within the chamber 204, and in acombustion profile engineered for superior work output, complete fueloxidation (and thus lesser soot), and controlled temperature (and thuslesser NOx). However, this RCCI methodology did not work well at lowengine loads (IMEP less than approximately 4 bar) and at idling (IMEPless than approximately 1 bar), where the engine tended to require theuse of only the high-reactivity fuel (e.g., diesel), and tended to havework and emissions outputs typical of conventional diesel operation.However, it was determined that if the pressure in the combustionchamber is below ambient air pressure at the start of the compressionstroke—i.e., if air intake is restricted during the intakestroke—conventional RCCI performance and benefits, as described above,can be attained at low load and idling conditions.

There are two preferred methods for achieving the desired sub-ambientpressure. Initially, a throttle 228 can be provided upstream from theintake port 210, with the throttle 228 being adjustable between an openstate (a state allowing maximum airflow from the intake manifold 208 tothe intake port 210) and a closed state (a state allowing minimumairflow from the intake manifold 208 to the intake port 210). Thethrottle 228 can be at least partially closed during at least the intakestroke of the engine combustion cycle, such that the intake manifoldpressure during this time—which is usually at ambient atmosphericpressure if no throttling is used—is negative owing to the suction ofthe intake stroke. Thus, the throttle is used to restrict the airentering the combustion chamber, and thereby adapt the equivalence ratioin the combustion chamber to a level such that RCCI methods can be usedeffectively. It is notable that in conventional diesel engines,throttles are generally not provided, or where they are provided, theyare used for exhaust gas remediation (e.g., for regeneration ofparticulate traps or for control of exhaust gas recirculation) ratherthan being used for combustion control.

As an alternative (or addition) to throttling upstream from the intakeport 210, variable valve actuation can also be used to restrict airflowto the combustion chamber 204, whereby the intake valve 212 is at leastpartially closed during one or more portions of the intake stroke (e.g.,opened late and/or closed early). As a result, throttling effectivelyoccurs at the intake port 210.

Variable valve actuation can also yield similar results if the intakevalve 212 is at least partially open during one or more portions of thecompression stroke, e.g., where the intake valve 212 is closed shortlyafter the compression stroke begins. In this instance, part of thecontents of the combustion chamber 204 is pushed out the intake port 210during compression. With appropriate fuel charge timing, amount, andcontent, the contents of the combustion chamber 204 can be made toresemble those where the intake valve 212 is at least partially closedduring one or more portions of the intake stroke. More specifically, byexpelling a portion of the air in the combustion chamber 204 so that theamount present during (the remainder of) compression resembles theamount of air that would be present if throttling was used, and byappropriate tailoring of the fuel charge(s), the combustion chambercontents can be made to attain the desired equivalence ratio. A similarresult could also be obtained if the exhaust valve 214 is opened duringpart of the compression stroke, but this is not recommended because aportion of the injected fuel charge(s) will be lost to the exhaust,decreasing efficiency and increasing emissions.

The equivalence ratio may also be adapted as desired by throttling at ordownstream from the exhaust port 216, as by appropriate timing of theopening/closing of the exhaust valve 214, and/or by including a throttlein the exhaust manifold). By increasing exhaust back pressure viaexhaust manifold throttling, and/or by retaining a small amount ofexhaust within the combustion chamber via appropriate timing of theintake valve 212 and/or the exhaust valve 214 (as by opening the intakevalve 212 or closing the exhaust valve 214 during a portion of theexhaust stroke), the amount of air within the combustion chamber (andthe equivalence ratio) can also be tailored to approximate thesub-ambient conditions that would be attained via intake throttling.

The throttling and valve actuation methods may be used alone or incombination during low load operation, i.e., the desired sub-ambientchamber pressure can be achieved via throttling, variable valveactuation, or via both of these methods, used at either or both of theintake and exhaust sides of the engine, and used in sequence orsimultaneously.

To summarize, in a preferred version of the invention, under moderateand higher loads (e.g., above about 4 bar IMEP), the engine of FIG. 2can be operated with a wide-open throttle, and without intake valverestriction, to perform the RCCI methods discussed in the prior patentapplication (and with the combustion chamber at or above ambientpressure at the start of the compression stroke). However, at low loadand no load (idling) conditions, airflow to the combustion chamber canbe restricted during at least the intake stroke to attain a sub-ambientchamber pressure sufficient to allow use of the RCCI methods.

It should be understood that when it is said that throttling can beused, the throttle can assume the form of a conventional throttleplate/blade which pivots into the airflow path prior to or within theintake manifold (in the manner of a butterfly valve), or it may take theform of other types of variable restrictions situated along the airintake path (e.g., in forms resembling gate valves or other types ofvalves or restrictions).

It is emphasized that the versions of the invention discussed above aremerely exemplary, and the invention can be modified in numerousrespects. Initially, the low- and high-reactivity fuel charges are notlimited to the use of gasoline and diesel, or to the use of gasoline ordiesel with a reactivity-modifying additive, and a wide variety of otherfuels (with or without additives) might be used instead (with ethanolbeing an example). The reactivity of a fuel can also be modified bymeans other than by the addition of an additive (or another fuel), as byaltering a fuel's composition, and/or by separating a fuel into lower-and higher-reactivity components, by use of devices on-board a vehiclefor cracking, heating, distilling, and/or catalysis along a vehicle'sfuel line. Reactivity can also be effectively modified by use of EGR(Exhaust Gas Recirculation) or similar measures, since recirculatedexhaust gas can hinder combustion.

As another example, the invention is not limited to the use of only twoor three fuel charges, e.g., four or more charges could be used.Further, the invention is not limited to the use of only two levels ofreactivity; to illustrate, each of three or more fuel charges may havedifferent reactivity than the other charges. In addition, fuels need notbe liquid in form, and gaseous fuels (such as methane/natural gas) mightbe used.

The invention is also compatible with EGR (Exhaust Gas Recirculation),as noted above, as well as exhaust after-treatment and other combustionmanipulation and emissions reduction strategies. These strategies mightreduce emissions even further, and since the emissions resulting fromthe invention are decreased from those in prior systems, the equipmentused to implement the strategies might have longer operating life,and/or may be modified for lesser expense.

In summary, the invention is not intended to be limited to the preferredversions of the invention described above, but rather is intended to belimited only by the claims set out below. Thus, the inventionencompasses all different versions that fall literally or equivalentlywithin the scope of these claims.

1. A compression ignition combustion method for an internal combustionengine having: a. a combustion chamber, b. an intake manifold; c. anintake port downstream from the intake manifold and upstream from thecombustion chamber, the intake port having an intake valve situatedtherein; d. a first tank containing a fuel having a first reactivity; e.an injector situated to supply the fuel into the combustion chamber; andf. a second tank containing a material having a second reactivity; themethod including the step of supplying both the fuel from the first tankand the material from the second tank to the combustion chamber duringan engine combustion cycle when the engine is idling.
 2. The method ofclaim 1 wherein the fuel from the first tank and the material from thesecond tank are supplied to the combustion chamber during the enginecombustion cycle to obtain a stratified distribution of fuel reactivitywithin the combustion chamber, with regions of highest fuel reactivitybeing spaced from regions of lowest reactivity.
 3. The method of claim 1wherein the fuel from the first tank and the material from the secondtank are supplied to the combustion chamber at different times duringthe engine combustion cycle.
 4. The method of claim 1 wherein: a. thefuel from the first tank is gasoline, and b. the material from thesecond tank is diesel fuel.
 5. The method of claim 1 wherein thematerial from the second tank has a reactivity greater than that ofdiesel fuel.
 6. The method of claim 5 wherein the method includes thestep of mixing the fuel from the first tank and the material from thesecond tank before the material from the second tank is supplied to thecombustion chamber.
 7. The method of claim 1 wherein the air pressure inthe combustion chamber is below ambient air pressure at the start of thecompression stroke of the engine combustion cycle.
 8. The method ofclaim 1 wherein the air pressure in the intake manifold is below ambientair pressure during the intake stroke of the engine combustion cycle. 9.The method of claim 1 wherein: a. the internal combustion engine alsohas a throttle upstream from the intake port, the throttle beingadjustable between: i. an open state allowing maximum airflow from theintake manifold to the intake port, and ii. a closed state allowingminimum airflow from the intake manifold to the intake port; and b. thethrottle is out of the open state during the intake stroke of the enginecombustion cycle.
 10. The method of claim 1 wherein: a. the intake valveis adjustable between: i. an open state allowing maximum airflow throughthe intake port, and ii. a closed state allowing no airflow through theintake port; and b. the intake valve is at least substantially in theclosed state before the end of the intake stroke of the enginecombustion cycle, whereby the air pressure in the combustion chamber isbelow ambient air pressure at the start of the compression stroke of theengine combustion cycle.
 11. The method of claim 1 wherein: a. theintake valve is adjustable between: i. an open state allowing maximumairflow through the intake port, and ii. a closed state allowing noairflow through the intake port; and b. the intake valve is closer tothe closed state than the open state during the intake stroke of theengine combustion cycle.
 12. The method of claim 1 wherein: a. theintake valve is adjustable between: i. an open state allowing maximumairflow through the intake port, and ii. a closed state allowing noairflow through the intake port; and b. the intake valve is out of theclosed state during a portion of the compression stroke of the enginecombustion cycle.
 13. The method of claim 1: a. wherein the internalcombustion engine additionally has an exhaust port downstream from thecombustion chamber, the exhaust port having an exhaust valve therein; b.further including the step of retaining a portion of the contents of thecombustion chamber upstream from the exhaust port during the enginecombustion cycle, whereby the portion of the combustion chamber contentsare present within the combustion chamber during a subsequent enginecombustion cycle.
 14. A compression ignition combustion method for aninternal combustion engine having: a. a combustion chamber, b. an intakemanifold opening onto the combustion chamber at an intake port, and c.an intake valve situated in the intake port, the method including thesteps of: (1) supplying an initial fuel charge into the combustionchamber; (2) thereafter supplying a subsequent fuel charge into thecombustion chamber, the subsequent fuel charge having a differentreactivity than the first fuel charge, wherein the air pressure in thecombustion chamber is below ambient air pressure at the start of thecompression stroke of the engine combustion cycle.
 15. The method ofclaim 13: a. wherein the internal combustion engine additionally has anexhaust port opening onto the combustion chamber, the exhaust porthaving an exhaust valve therein; b. further including the steps of: i.at least partially combusting the fuel charges within the combustionchamber during the engine combustion cycle, thereby generating exhaustgas; and ii. retaining a portion of the exhaust gas upstream from theexhaust port during the engine combustion cycle, whereby the portion ofthe exhaust gas is present within the combustion chamber during at leastthe intake stroke of a subsequent engine combustion cycle.
 16. Themethod of claim 13 wherein the air pressure in the intake port is belowambient air pressure at least during the intake stroke of the enginecombustion cycle.
 17. The method of claim 13 wherein the engine isidling.
 18. The method of claim 13 wherein the engine is operating withan indicated mean effective pressure of less than 4 bar.
 19. The methodof claim 13 wherein the initial fuel charge, if fully dispersed withinthe combustion chamber, results in an equivalence ratio of 0.2 orgreater within the combustion chamber.
 20. The method of claim 13wherein the subsequent fuel charge is supplied to the combustion chamberduring the engine combustion cycle to obtain a stratified distributionof fuel reactivity within the combustion chamber, with regions ofhighest fuel reactivity being spaced from regions of lowest reactivity.21. The method of claim 13 wherein: a. the internal combustion enginealso has: i. a first tank containing a fuel having a first reactivity;and ii. a second tank containing a material having a second reactivity;b. one of the initial and the subsequent fuel charges contains the fuelfrom the first tank; c. the other of the initial and the subsequent fuelcharges contains the fuel from the first tank and the material from thesecond tank.
 22. The method of claim 13 wherein the initial andsubsequent fuel charges are each supplied from separate tanks whichsupply one or more injectors situated to supply the fuel charges intothe combustion chamber.
 23. The method of claim 13 wherein: a. theinternal combustion engine also has a throttle upstream from the intakeport, the throttle being adjustable between: i. an open state allowingmaximum airflow from the intake manifold to the intake port, and ii. aclosed state allowing minimum airflow from the intake manifold to theintake port; and b. the throttle is out of the open state during theintake stroke of the engine combustion cycle.
 24. The method of claim 13wherein: a. the intake valve is adjustable between: i. an open stateallowing maximum airflow through the intake port, and ii. a closed stateallowing no airflow through the intake port; and b. the intake valve isat least substantially in the closed state during at least a portion ofthe intake stroke of the engine combustion cycle.
 25. The method ofclaim 13 wherein: a. the intake valve is adjustable between: i. an openstate allowing maximum airflow through the intake port, and ii. a closedstate allowing no airflow through the intake port; and b. the intakevalve is out of the closed state during at least a portion of thecompression stroke of the engine combustion cycle.
 26. A compressionignition combustion method for an internal combustion engine having: a.a combustion chamber, b. an intake manifold opening onto the combustionchamber at an intake port, c. an intake valve situated in the intakeport, d. a throttle upstream from the intake port, the throttle beingadjustable between: i. an open state allowing maximum airflow from theintake manifold to the intake port, and ii. a closed state allowingminimum airflow from the intake manifold to the intake port; the methodincluding the steps of: (1) supplying an initial fuel charge into thecombustion chamber; (2) thereafter supplying a subsequent fuel chargeinto the combustion chamber, the subsequent fuel charge having adifferent reactivity than the first fuel charge, during an enginecombustion cycle, wherein the throttle is between the open and closedstates during the cycle.
 27. The method of claim 24 wherein the engineis idling.
 28. The method of claim 24 wherein the engine is operatingwith an indicated mean effective pressure of less than 4 bar.
 29. Themethod of claim 24 wherein the subsequent fuel charge is supplied to thecombustion chamber during the engine combustion cycle to obtain astratified distribution of fuel reactivity within the combustionchamber, with regions of highest fuel reactivity being spaced fromregions of lowest reactivity.
 30. The method of claim 24 wherein: a. theintake valve is adjustable between: i. an open state allowing maximumairflow through the intake port, and ii. a closed state allowing noairflow through the intake port; and b. the intake valve is one or moreof: i. at least substantially in the closed state during at least aportion of the intake stroke of the engine combustion cycle, and ii. outof the closed state during at least a portion of the compression strokeof the engine combustion cycle.
 31. The method of claim 24 wherein: a.the internal combustion engine also has: i. a first tank containing afuel having a first reactivity; and ii. a second tank containing amaterial having a second reactivity; b. one of the initial and thesubsequent fuel charges contains the fuel from the first tank; c. theother of the initial and the subsequent fuel charges contains the fuelfrom the first tank and the material from the second tank.
 32. Themethod of claim 24 wherein the initial and subsequent fuel charges areeach supplied from separate tanks which supply one or more injectorssituated to supply the fuel charges into the combustion chamber.